1. Can I make the column title only 1 row. i.e., C1: Trial Balance Debit and D1 Trial Balance credit and so on.

Yes, that method will work well

2. Is it advisable to fill the blank cells something like (NA)?

No, That is not necessary. The real issue is using blank rows and columns for formatting. A single blank data cells are fine. When you have blank rows and columns it is very difficult to determine the edges of a table in a worksheet.

I am in a project to make the Excel files compliant with the Section 508.
I am in confusion of the following:
1.15: If there are tables, are blank cells avoided?
2.1: Is the table free of merged cells? (If not, but the final format will be PDF or HTML, then merged cells are okay)

The sheets are accounting files where the column titles go for 2 rows.
i.e., C1 is Trial Balance, D1 is blank and E1 is adjustment. Then in the next row C2 is debit and D2 is Credit, E2 is Debit and F2 is Credit.

1. In the above scenario what is the best method without merging C1 and D1?
2. The entry will be either for Debit or Credit. How to avoid blank cells?
3. Is there any specific tutorial to make the Excel files accessible?

Few Thoughts:
1. Can I make the column title only 1 row. i.e., C1: Trial Balance Debit and D1 Trial Balance credit and so on.
2. Is it advisable to fill the blank cells something like (NA)?

Note: As per the accounting rules I don't think the above things can be encorporated.